The detection and/or quantitation of specific nucleic acid sequences is an increasingly important technique for identifying and classifying microorganisms, diagnosing infectious diseases, detecting and characterizing genetic abnormalities, identifying genetic changes associated with cancer, studying genetic susceptibility to disease, and measuring response to various types of treatment. Such procedures have also found expanding uses in detecting and quantitating microorganisms in foodstuffs, environmental samples, seed stocks, and other types of material where the presence of specific microorganisms may need to be monitored. Other applications are found in the forensic sciences, anthropology, archaeology, and biology where measurement of the relatedness of nucleic acid sequences has been used to identify criminal suspects, resolve paternity disputes, construct genealogical and phylogenetic trees, and aid in classifying a variety of life forms.
A common method for detecting and quantitating specific nucleic acid sequences is nucleic acid hybridization. This method is based on the ability of two nucleic acid strands that contain complementary or essentially complementary sequences to specifically associate, under appropriate conditions, to form a double-stranded structure. To detect and/or quantitate a specific nucleic acid sequence (known as the "target sequence"), a labelled oligonucleotide (known as a "probe") is prepared that contains sequences complementary to those of the target sequence. The probe is mixed with a sample suspected of containing the target sequence, and conditions suitable for hybrid formation are created. The probe hybridizes to the target sequence. If it is present in the sample. The probe-target hybrids are then separated from the single-stranded probe in one of a variety of ways. The amount of label associated with the hybrids is then measured as an indication of the amount of target sequence in the sample.
The sensitivity of nucleic acid hybridization assays is limited primarily by the specific activity of the probe, the rate and extent of the hybridization reaction, the performance of the method for separating hybridized and unhybridized probe, and the sensitivity with which the label can be detected. The most sensitive procedures may lack many of the features required for routine clinical and environmental testing such as speed, convenience, and economy. Furthermore, their sensitivities may not be sufficient for many desired applications.
As a result of the interactions among the various components and component steps of this type of assay, there is almost always an inverse relationship between sensitivity and specificity. Thus, steps taken to increase the sensitivity of the assay (such as increasing the specific activity of the probe) may result in a higher percentage of false positive test results. The linkage between sensitivity and specificity has been a significant barrier to improving the sensitivity of hybridization assays. One solution to this problem would be to specifically increase the amount of target sequence present using an amplification procedure. Amplification of a unique portion of the target sequence without amplification of a significant portion of the information encoded in the remaining sequences of the sample could give an increase in sensitivity while at the same time not compromising specificity.
A method for specifically amplifying nucleic acid sequences termed the "polymerase chain reaction" or "PCR" has been described by Mullis et al. (See U.S. Pat. Nos. 4,683,195, 4,683,202 and 4,800,159 and European patent applications 86302298.4, 86302299.2, and 87300203.4 and Methods in Enzymology, Volume 155, 1987, pp. 335-350.) The procedure uses repeated cycles of primer dependent nucleic acid synthesis occurring simultaneously using each strand of a complementary sequence as a template. The sequence that is amplified is defined by the locations of the primer molecules that initiate synthesis. The primers are complementary to the 3'-end portion of the target sequence or its complement and must complex with those sites in order for nucleic acid synthesis to begin. After extension product synthesis, the strands are separated, generally by thermal denaturation, before the next synthesis step. In the PCR procedure, copies of both strands of a complementary sequence are synthesized.
The strand separation step used in PCR to separate the newly synthesized strands at the conclusion of each cycle of the PCR reaction is often thermal denaturation. As a result, either, a thermostable enzyme is required or new enzyme must be added between thermal denaturation steps and the initiation of the next cycle of DNA synthesis. The requirement of repeated cycling of reaction temperature between several different and extreme temperatures is a disadvantage of the PCR procedure. In order to make the PCR convenient, programmable thermal cycling instruments are required.
The PCR procedure has been coupled to RNA transcription by incorporating a promoter sequence into one of the primers used in the PCR reaction and then, after amplification by the PCR procedure for several cycles, using the double-stranded DNA as template for the transcription of single-stranded RNA. (See, e.g., Murakawa et al., DNA 7:287-295 (1988).)
Other methods for amplification of a specific nucleic acid sequence comprise a series of primer hybridization, extending and denaturing steps to provide an intermediate double stranded DNA molecule containing a promoter sequence through the use of a promoter sequence-containing primer. The double stranded DNA is used to produce multiple RNA copies of the target sequence. The resulting RNA copies can be used as target sequences to produce further copies, and multiple cycles can be performed. (See, e.g., Burg, et al., WO 89/1050; Gingeras, et al., WO 88/10315 (sometimes called "transcription amplification system" or TAS); EPO Application No. 89313154 to Kacian and Fultz; EPO Application No. 88113948.9 to Davey and Malek; Malek, et al. W091/02818.)
Walker, et al., Prod. Natl. Acad. Sci. (USA) 89:392-396 (Jan. 1992), not admitted to be prior art, describes an oligonucleotide driven amplification method for use with a DNA template, using a restriction endonuclease to produce the initial target sequences and an enzyme to nick the DNA/DNA complex in order to enable an extension reaction and therefore amplification. Becker, et al., EPO Application.No. 88306717.5, describes an amplification method in which a primer is hybridized to the target sequence and the resulting duplex is cleaved prior to the extension reaction and amplification; in the case where the primer extends past the region of hybridization, it requires cleavage prior to the extension and the primer must be blocked at its 3'-end to prevent any unwanted extension reactions from occurring prior to amplification. Urdea, WO 91/10746, describes a signal amplification method that incorporates a T7 promoter sequence.
Other methods of amplifying nucleic acid include the ligase chain reaction (LCR), described in European Patent Application No. 320,308, in which at least four separate oligoprobes are used; two of the oligoprobes hybridize to opposite ends of the same target strand in appropriate orientation such that the third and fourth oligoprobes may hybridize with the first and second oligoprobes to form, upon ligation, connected probes that can be denatured and detected. Another method is that described in EPO Application No. 0 427 073 A2, published May 15, 1991 and not admitted to be prior art, in which a palindromic probe able to form a hairpin and having a functional promoter region in the hairpin is hybridized to a target sequence, then ligated to another oligonucleotide hybridized to the target sequence such that specific RNA transcripts may be made.
Relatively large amounts of certain RNAs may be made using a recombinant single-stranded RNA molecule having a recognition sequence for the binding of an RNA-directed polymerase, preferably Q.beta. replicase. (See, e.g., U.S. Pat. No. 4,786,600 to Kramer, et al.) A number of steps are required to insert the specific sequence into a DNA copy of the variant molecule, clone it into an expression vector, transcribe it into RNA and then replicate it with Q.beta. replicase.
Definitions
As used herein, the following terms have the following meanings unless expressly indicated to the contrary.
A. Nucleic Acid
"Nucleic acid" means either RNA or DNA, along with any nucleotide analogues or other molecules that may be present in the sequence and that do not prevent performance of the present invention.
B. Template
A "template" is a nucleic acid molecule that is able to be copied by a nucleic acid polymerase. A template may be either RNA or DNA, and may be any of single-stranded, double-stranded or partially double-stranded, depending on the polymerase. The synthesized copy is complementary to the template. In this invention, the term copies also includes nucleic acid having the equivalent RNA or DNA sequence to a template, which are commonly referred to as homologous sequences in the art.
C. Primer
A "primer" is an oligonucleotide that is complementary to a template that hybridizes with the template to give a primer/template complex for initiation of synthesis by a DNA polymerase, such as a reverse transcriptase, and which is extended by the addition of covalently bonded bases linked to its 3' end that are complementary to the template. The result is a primer extension product. Virtually all DNA polymerases (including reverse transcriptases) that are known require complexing of an oligonucleotide to a single-stranded template ("priming") to initiate DNA synthesis. Under appropriate circumstances, a primer may be a part of a promoter-primer. Such primers are generally between 10 and 100 bases in length, preferably between 20 and 50 bases in length.
D. Promoter or Promoter Sequence
A "promoter" or "promoter sequence" is a specific nucleic acid sequence that is recognized by a DNA-dependent RNA polymerase ("transcriptase") as a signal to bind to a nucleic acid molecule and begin the transcription of RNA at a specific site. For binding, such transcriptases generally require that the promoter and its complement be double-stranded; the template portion need not be double-stranded. Individual DNA-dependent RNA polymerases recognize a variety of different promoter sequences that can vary markedly in their efficiency of promoting transcription. When an RNA polymerase binds to a promoter sequence to initiate transcription, that promoter sequence is not part of the sequence transcribed. Thus, the RNA transcripts produced thereby will not include the promoter sequence.
E. Promoter-primer
A promoter-primer comprises a promoter and a primer. It is an oligonucleotide that is sufficiently complementary to the 3'-end of a target nucleic acid sequence to complex at or near the 3'-end of that target nucleic acid sequence, which means that the promoter-primer complexes near enough the end of the target sequence to allow amplification of enough of the target sequence that the requirements of the assay, testing, cloning or other use for the amplified nucleic acid are met. The promoter-primer is used as a template to create a complementary nucleic acid sequence extending from the 3'-end (also known as the 3' terminus) of a target nucleic acid sequence, to result in a generally double stranded promoter, subject to any denaturing or enzymatic activity that may disrupt the double strand. Such promoter-primers are generally between 40 and 100 bases in length, preferably between 40 and 60 bases.
A DNA- or RNA-dependent DNA polymerase also creates a complementary strand to the target nucleic acid molecule, using the target sequence as a template.
F. Modified Primer or Promoter-primer
The 3'-end of the primer or promoter-primer may be modified, or blocked, so as to prevent or reduce the rate and/or extent of an extension reaction from proceeding therefrom. A primer or promoter-primer having both modified and unmodified members consists of essentially the same nucleic acid sequence for the purposes of the present invention. In other words, the modified primer or promoter-primer does not contain a different complexing sequence (primer) in that both the modified and unmodified oligonucleotide hybridize in effectively the same position (plus or minus about ten bases) on the target nucleic acid sequence. Also, the modified promoter-primer does not contain a different recognition sequence (promoter) from the unmodified promoter-primer. This means that, within about 10 bases, the modified and unmodified primers or promoter-primers are the same, are recognized by the same RNA polymerase, and hybridize to more or less the same target sequence (although not necessarily at precisely the same position). In a preferred embodiment, the modified and unmodified primers or promoter-primers are identical except for the modification.
The 3'-end of the target complementary portion of a primer or promoter-primer can be modified in a variety of ways well known to those skilled in the art. Appropriate modifications to a promoter-primer can include addition of ribonucleotides, 3' deoxynucleotide residues, (e.g., cordycepin (CO, Glen Research)), 3', 2'-dideoxy nucleotide residues, modified nucleotides with nonphosphodiester backbone linkages (such- as phosphorothioates), and non-nucleotide linkages such as described in Arnold, et al., (PCT US 88/03173) (RS) or alkane-diol modifications (Wilk et al. Nuc. Acids Res. 18:2065, 1990) (RP), or the modification may simply, consist of one or more nucleotide residues 3' to the hybridizing sequence that are uncomplementary to the target nucleic acid. Of course, other effective modifications are possible as well.
A mixture of modified and unmodified oligonucleotides may be used in an amplification reaction, and a broad range of ratios of modified to unmodified oligonucleotide (e.g., from 1:1 to 1,000:1) can be used. A mixture of oligonucleotides with different 3' modifications may also be used.
G. Plus (+) and Minus (-) Strand(s)
Discussions of nucleic acid synthesis are greatly simplified and clarified by adopting terms to name the two complementary strands of a nucleic acid duplex. Traditionally, the strand encoding the sequences used to produce proteins or structural RNAs was designated as the "plus" strand and its complement the "minus" strand. It is now known that in many cases, both strands are functional, and the assignment of the designation "plus" to one and "minus" to the other must then be arbitrary. Nevertheless, the terms are very useful for designating the sequence orientation of nucleic acids and will be employed herein for that purpose, with the "plus" strand denominating the original target sequence strand that is complexed with the first primer or promoter-primer.
H. Target Nucleic Acid Sequence, Target Sequence
A "target nucleic acid sequence," or "target sequence," has a desired nucleic acid sequence to be amplified, and may be either single-stranded or double-stranded and may include other sequences 5' or 3' of the sequences to be amplified which may or may not be amplified.
The target nucleic acid sequence includes the complexing sequences to which the promoter-primer hybridizes during performance of the present invention. Where the target nucleic acid sequence is originally single-stranded, the term refers to either the (+) or (-) strand, and will also refer to the sequence complementary to the target sequence. Where the target nucleic acid sequence is originally double-stranded, the term refers to both the (+) and (-) strands.
I. DNA-Dependent DNA Polymerase
A "DNA-dependent DNA polymerase" is an enzyme that synthesizes a complementary DNA copy from a DNA template. An example is bacteriophage T7 DNA polymerase. All known DNA-dependent DNA polymerases require a complementary primer, which can be RNA or DNA, or a copolymer, to initiate synthesis. It is known that under suitable conditions certain DNA-dependent DNA polymerases may synthesize a complementary DNA copy from an RNA template.
J. DNA-Dependent RNA Polymerase (Transcriptase)
A "DNA-dependent RNA polymerase" or "transcriptase" is an enzyme that synthesizes multiple RNA copies from a double-stranded or partially-double stranded DNA molecule having a (usually double-stranded) promoter sequence. It should be noted that the present invention includes single stranded promoter sequences in the promoter-primer, along with the RNA polymerases that recognize them. The RNA molecules ("transcripts") are synthesized in the 5'.fwdarw.3' direction of the RNA molecule, beginning at a specific position just downstream of the promoter. Examples of transcriptases are the DNA-dependent RNA polymerases from bacteriophages T7, T3, and SP6.
K. RNA-Dependent DNA Polymerase (Reverse Transcriptase)
An "RNA-dependent DNA polymerase" or "reverse transcriptase" is an enzyme that synthesizes a complementary DNA copy from an RNA template. All known reverse transcriptases also have the ability to make a complementary DNA copy from a DNA template; thus, they are both RNA- and DNA-dependent DNA polymerases. A primer is required to initiate synthesis with either the RNA or DNA templates.
L. RNAse H
An "RNAse H" is an enzyme that degrades the RNA portion of an RNA:DNA duplex. RNAse H's may be endonucleases or exonucleases. Avian myeloblastosis virus and Moloney murine leukemia virus reverse transcriptases contain an RNAse H activity in addition to their polymerase activity. Some cloned reverse transcriptases lack RNAse H activity. There are also sources of RNAse H available without an associated polymerase activity. The degradation may result in separation of RNA from an RNA:DNA complex. Alternatively, the RNAse H may simply cut the RNA at various locations such that portions of the RNA melt off or permit enzymes to unwind portions of the RNA, or the RNA fragments generated may serve as primers for extension by a polymerase.
M. Hybridize, Complex
The terms "hybridize" and "complex" refer to the formation of duplexes between nucleotide sequences that are sufficiently complementary to form duplexes (or "complexes") via Watson-Crick base pairing. Where a promoter-primer or primer "hybridizes" with target (template), such complexes (or hybrids) are sufficiently stable to serve the priming function required by a DNA polymerase to initiate DNA synthesis.
N. Specificity
Specificity is a characteristic of a nucleic acid sequence that describes its ability to distinguish between target and non-target sequences, dependent on sequence and assay conditions.